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SprayMon

SprayMon allows players to see authors of sprays.

Don't just press "subscribe", it won't work. This addon is for servers. See instructions at the bottom.

"spraymon" console command will show you sprays of all players on server.

Change "spraymon_nodelay" convar to set who can spray without delay: 0 - nobody | -1 - everyone | 1 - admins | 2 - superadmins. By default it's 0.
If you are using ULX, you can give "spraymon_nodelay" access to group or user. You don't have to change convar for that.

Set "spraymon_nooverspraying" convar to 1 to block ability to draw sprays over other sprays.
Check is performed on client, and current visible sprays are different on different clients, so this works only 50% of times.

You can set convars by, for example, editing server.cfg

Hold ALT for SprayMon to show SteamIDs and copy first one to clipboard.

This addon is for servers, it's pointless on clients.
How to install workshop addons to dedicated servers:
http://wiki.garrysmod.com/page/Workshop_for_Dedicated_Servers
or
http://facepunch.com/showthread.php?t=1242185